Mass analyzer with extension means to decrease the distance between electrode surfaces



Oct. 14, 1969 w. M. BRUBAKER MASS ANALYZER WITH EXTENSION MEANS TODECREASE THE DISTANCE BETWEEN ELECTRODE SURFACES Filed June 19, 1967INVENTOR VV/Lifl/V Mia/5411452 ma 1% ATTDE/VEV? United States Patent MUS. Cl. 250-413 Claims ABSTRACT OF THE DISCLOSURE A non-magnetic massanalyzer with improved transmission efiiciency. In this analyzer thedistance between a field electrode or electrodes of the analyzer in thevicinity of the entrance aperture is made smaller than the distancebetween the electrodes in that portion of the analyzer more remote fromthe aperture.

The invention described herein was made in the performance of work undera NASA contract and is subject to the provisions of Section 305 of theNational Areonautics and Space Act of 1958, Public Law 85-568 (72 Stat.435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION This invention relates to a non-magnetic masanalyzer and in particular to a mass filter having specially constructedfield-forming electrodes to improve transmission etficiency.

In the operation of non-magnetic analyzers such as monopole andquadrupole mass filters (described in US. Patents 3,197,633 and2,939,952 respectively), it has been found that exposure of chargedparticles to be analyzed to the fringing electric fields existing at theentrance end of the analyzer for more than two or three cycles of the ACvoltage connected to the field-forming electrodes results in anundesirable radial impulse being imparted to the particles. If theimpulse is sufiiciently high, the charged particles contact and aredischarged at the electrodes of the analyzer reducing the quantity ofcharged particles which would otherwise be transmitted by the analyzerand creating several other undesirable side effects.

In order to reduce the impulse producing efiect of the fringing held, afirst approach has been to impart a relatively high injection energy toparticles as they are directed toward the entrance end of the filter soas to reduce the transit time of the particles through the fringingfields to a minimum. A high injection energy, however, limits themaximum power which can be obtained and imposes additional electricpower requirements on the instrument. To restore some of the lostresolving power an analyzer of increased length is often provided inorder to permit the analyzing or resolving action of the electric fieldswithin the analyzer to be exerted over a longer distance and particletransit time. Increased analyzer length is frequently undesirableespecially when the analyzer is made part of the instrumentation in aspace vehicle.

A second approach has been to provide auxiliary electrodes adjacent theentrance end of the instrument. The ratio of the voltages connected tothese electrodes is then arranged such that particles entering theanalyzer encounter an intermediate ratio in the transition from theregion of zero field outside the analyzer to the region of very strongelectric fields in the center of the analyzer. This is done by reducingthe amplitude of the DC voltage connected to the auxiliary electrodeswhile leaving the amplitude of the AC voltage unchanged. By this means asubstantial increase in the transmission elficiency of 3,473,019Patented Oct. 14, 1969 the analyzer is achieved. Such an approach isdescribed in US. Patent 3,129,327.

SUMMARY OF THE INVENTION The present invention provides another approachto the reducing of the etfect of the fringing field at the entrance to anon-magnetic mass analyzer. The invention provides a non-magnetic massanalyzer comprising at least one elongated field-forming electrodehaving a longitudinal axis and a second elongated electrode extendinglongitudinally of and parallel to the field-forming electrode such thatthe two or more electrodes define an analyzing region there-between, theanalyzing region having a longitudinal axis, an entrance end and an exitend. A source of AC voltage and a source of DC voltage are connected tothe field-forming electrode to create a mass analyzing electric fieldbetween the two electrodes. A source of charged particles is located atthe entrance end of the analyzing region and means for detecting chargedparticles is located at the exit end of the analyzing region. Extensionmeans are located at the entrance end of the analyzer in an electricallyconductive relationship with the field-forming electrode for decreasingthe distance between the electrode surface along a line perpendicular tothe longitudinal axis of the analyzing region.

By narrowing the distance between the electrodes of a mass analyzer atthe entrance end of the instrument, the analyzing electric field nowreaches full strength at a position closer to the entrance end of theanalyzer. Charged particles entering the analyzer therefore traverse ashorter distance measured along the axis of the analyzer through anelectric field of reduced strength, i.e., of the so-called fringingelectric field. This reduction in exposure to the fringing field resultsin a diminution of the radial impulse which would normally be impartedto particles as they pass through such a field.

Reduction of the elfect of fringing fields at the entrance of theanalyzer in the manner of the present invention permits severaladvantageous changes in the structure and operating parameters of theanalyzer. The length of the analyzer can be reduced and the injectionenergy can be lowered 'without a reduction in resolving power orsensitivity. Alternatively, an analyzer having the same overall lengthas before can be operated at a lower frequency and lower power withoutany significant degradation of performance.

DESCRIPTION OF THE DRAWINGS The above advantages and others will bebetter understood by reference to the following figures in which:

FIG. 1 is a schematic diagram in perspective of a conventionalquadrupole mass analyzer;

FIG. 2 is a view of a pair of electrodes of a quadrupole mass analyzerincorporating the improvement of the present invention;

FIG. 3 is a view taken along lines 33 of FIG. 2;

FIG. 4 is an alternate embodiment of the invention as incorporated in aquadrupole mass analyzer.

FIG. 5 is a second alternate embodiment of the invention as incorporatedin a monopole mass analyzer; and

FIG. 6 is a stability diagram for a quadrupole mass analyzer.

In FIG. 1 is depicted a schematic illustration of a conventionalquadrupole mass analyzer. The analyzer includes four elongatedfield-forming electrodes 10 in the form of cylindrical rods disposed atintervals about a longitudinal axis 12, designtaed the Z axis.Completing the Cartesian coordinate frame of reference, an X axis 14 anda Y axis 16 lie in the plane of the entrance end of the rod electrodes.The rods lying in the X-plane are referred to as the X or positive rodsand the rods lying in the Y-plane are resignated the Y or negative rods.A

source of AC voltage 18and a source of DC voltage 20 are connected tothe X- and Y-rods, respectively, such that the positive pole of the DCsource is connected to the X-rods and the negative pole of the DC sourceto the Y-rods. Connection of sources 18 and 20 to the electrodes causesa multipole electric field analyzing region with a longitudinal axis 12to be created within the analyzer.

In operation the rods are enclosed in a housing (not shown) which isnormally evacuated and charged particles, normally ions, from a source11 are directed into the analyzer longitudinally of the rods andgenerally along the Z axis toward a detector located at the exit end ofthe electrodes. The charged particles injected into the analyzing fieldare then sorted with only particles of a mass to charge ratio determinedby the amplitude of the AC and DC voltages and the frequency of the ACvoltage being transmitted through the analyzer.

The operation of a quadrupole mass analyzer or filter is frequentlydescribed in terms of a diagram such as is shown in FIG. 6, a stabilitydiagram. The parameters indicated along the ordinate 48 and abscissa 50are proportional, respectively, to the amplitude of the DC and ACvoltages connected to the electrodes of the filter. For given specificvalues of AC and DC voltage, and frequency of AC voltage, thetransmission of ions through the analyzer can be termined by referenceto line 52 on the diagram, a line designated the scan line. The areaunder triangular shaped figure 54 is referred to as the stable portionof the diagram. The areas to the left and right outside the triangle arereferred to as the Y- and X- unstable portions respectively. Therefore,for the conditions shown in FIG. 6, ions in a narrow mass rangecorresponding to the points on line 52 just below the apex of thetriangle are the only masses transmitted by the analyzer.

The scan line 52 can also be referred to in connection with theconditions experienced by an ion which would normally be transmittedthrough the analyzer as it passes through the entrance fringing fields.Since the field strengths vary from zero to full value, as the iontraverses this region, ions having a transmissible M/e ratio can be saidto have unstable trajectories until they arrive in the region of fullfield strength, i.e., they correspond to ions having working pointswhich lie on that portion of the scan line in the Y-unstable portion ofthe stability diagram. In the Y unstable region such ions receive animpulse in the Y-direction away from the axis 12 of the analyzer. Asufiiciently strong impulse causes the ion to strike one of the rods andbe discharged.

The effect of the fringing field is reduced by shaping the entrance ofone or more of the field electrodes as shown in FIGS. 2 and 3. As showntherein the distance between the rods at the entrance end is decreasedalong a line perpendicular to the longitudinal axis 12 of the analyzingregion. Charged particles passing through entrance aperture 26 now movefrom an era of zero field strength adjacent entrance end 28 ofelectrodes 21 and 23 to an area of maximum field strength within theinterior of the analyzer over a shorter distance and in a shorter time.This reduction in the length of the fringing field along thelongitudinal axis of the analyzer means that the charged particlesentering the analyzer are exposed to fewer cycles of the AC voltageconnected to the electrodes of the analyzer (i.e., they move to thestable portion of the stability diagram in a shorter time span), therebyminimizing the radial impulse which is normally imparted.

Alternate embodiments to the invention depicted in FIGS. 2 and 3 areshown in FIGS. 4 and 5. In FIG. 4 a pair of rod electrodes 30 and 32 areprovided with discs 34 and 36 attached to the entrance end of theelectrodes in a conductive relationship therewith. A flat electrode 40completes the structure of a dualpole mass analyzer. As indicated above,the reduction in the length of the fringing field is achieved byreducing the distance between the electrodes of the mass analyzer at theentrance end of the analyzer. Thus the provision of discs at the end ofthe rods of an analyzer achieves this objective. Collar 38 disposedaround rod electrode 42 achieves the same result as shown in FIG. 5 withright angle electrode 44 completing the structure of a monopole massanalyzer. As the depth of the collar increases, the distance from collar38 to an axis 46 extending longitudinally of the electrodes 42decreases.

Although described primarily with reference to a uadrupole mass filter,the improvement of the present invention is also useful in massanalyzers of the monopole and dualpole type. By decreasing the distancebetween the field-forming electrode or electrodes at the entrance end ofthe analyzer, the length of the fringing field is reduced and thetransmission efficiency increased. The invention can also be describedin terms of the diameter of the rod electrodes of a mass analyzer. Ifthe diameter of the electrodes of a mass analyzer in the immediatevicinity of the entrance aperture is made larger than the diameter ofthe rest of the electrode, then the magnitude of the electric field nearthe axis of the instrument reach their full value at a position nearerthe entrance aperture. The exact shape and extent of the enlargement isnot critical. In the preferred embodiment modification of the entranceend of the rods should be symmetrical and the same for each rod. In aquadrupole the modification is preferably made to the Y or negative rodsonly since the radial impulse normally imparted is Y directed.

The same modification to the field-forming electrode can also beprovided with beneficial results at the exit end of the analyzer. Asimilar impulse producing fringing field also exists at this end of ananalyzer and reduction of its effect likewise improves the transmissionefficiency of the analyzer.

If the linear dimension of the fringing field along the axis isshortened, several advantages can be obtained. An analyzer which isshorter in length acting on charged particles introduced with a lowerinjection energy can achieve the same resolving power and sensitivity ofan analyzer having rods of uniform contour into which ions of higherinjection energy are directed. Alternatively, it is possible to operatean instrument of the same overall length which has been provided withthe improvement of this invention at a lower frequency and power Withoutdiminishing the resolving power or sensitivity. Put in another way, aninstrument made according to the improvement of the present inventionoutperforms a conventional instrument with all other parametersunchanged. It yields higher sensitivity at a given resolving power orhigher resolving power at the same sensitivity.

What is claimed is:

1. A non-magnetic mass analyser comprising:

at least one elongated field-forming electrode;

a second elongated electrode extending longitudinally of and parallel tothe field electrode;

a source of AC voltage;

a source of DC voltage;

means for connecting the sources of AC and DC voltage to the fieldelectrode for creating a mass analyzing field region between the twoelectrodes, the analyzing region having a longitudinal axis locatedbetween and substantially parallel to the two electrodes and an entranceend and exit end located at opposite ends of the longitudinal axisrespectively;

a source of charged particles located at the entrance end of theanalyzing region;

means for detecting charged particles located at the exit end of theanalyzing region; and

extension means located at the entrance end of the analyzing region inelectrically conductive relationship with the field-forming electrodefor decreasing the distance between the electrode surfaces along a lineperpendicular to the longitudinal axis of the analyzing region.

2. A mass analyzer according to claim 1 wherein the mass analyzer is ofthe monopole type with the elongated field-forming electrode having arod configuration and the second electrode having a right angleconfiguration bracketing the field-forming electrode, the extensionmeans being connected to the rod electrode.

3. A mass analyzer according to claim 1 wherein the mass analyzer is ofthe dualpole type with two fieldforming electrodes, each having a rodconfiguration and the second electrode having a flat configuration, theextension means being connected to at least one of the rod electrodes.

4. A mass analyzer according to claim 1 wherein second extension meansare located at the exit end of the analyzing region in electricallyconductive relationship with the field-forming electrode.

5. A mass analyzer according to claim 1 wherein the analyzer is of thequadrupole type comprising four fieldforming rod electrodessymmetrically disposed about a central axis, the second electrodecorresponding to the fourth field-forming electrode, a first pair ofopposed rods defining, according to the Cartesian coordinates frame ofreference, X rods lying along an X axis of the analyzer and the secondpair of opposed rods defining Y rods lying along a Y axis of theanalyzer, the extension means being connected to at least one of the rodelectrodes.

6. A mass analyzer according to claim 5 wherein the extension means areconnected to the Y rods only.

7. A mass analyzer according to claim 5 wherein the extension means areconnected to all four rods of the analyzer.

8. A mass analyzer according to claim 1 wherein the extension means is aconductive disc.

9. A mass analyzer according to claim 1 wherein the extension means is aconductive collar.

10. A mass analyzer according to claim 1 wherein a field-formingelectrode is provided in the form of a cylindrical rod and the extensionmeans is a contoured portion of the rod extending toward the secondelectrode.

WILLIAM F. LINDQUIST, Primary Examiner 23 2 33 UNITED S'IA'ILQS PATENTOFFICE CERTIFICATE OF CORRECTION Patent No. 3,473,019 Dated October 14,1969 Inventor(s) Wilson M. Brubaker It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Q01. 1, line 29, "mas" should read --mass--;

line 52, after "maximum" insert --resolving--.

Col. 2, line 24, "surface" should read --surfaces--;

line 58, "quadrupole" should read --dualp01e--; line 68, ,"designtaed"should read --designated--; line 73, "resignated" should read--designated-- Col. 3, line 26, "termined" should read --determined--;

line 50, after "entrance" insert --end--.

(301. 4, line 6, "electrodes" should read --electrode--.

SIGNED AND SEALED MAY 1 91970 Attcat:

Edward M. FletchnrJr- WILLIAM E. sum, .m. Amazing Officer onar 01Patent.

